1. Field of the Invention
The present invention relates to an electric power tool including a so-called outer rotor type motor in which a tubular rotor rotates around a stator.
2. Description of the Related Art
A prior art relating to an electric power tool including an outer rotor type motor as described above is disclosed in Japanese Patent Application Laid-Open No. 2006-150571.
The electric power tool disclosed in Japanese Patent Application Laid-Open No. 2006-150571 is an electric sander including an outer rotor type motor. By using an outer rotor type motor, the diameter of the rotor can be set large, and a large rotational torque can be obtained due to a high centrifugal force.
Generally, in an outer rotor type motor that is used in an electric power tool, a rotor 102 has four poles of a permanent magnet 103, and a stator 105 has six slots 106, as shown in FIG. 7.
In case where there are four poles of the permanent magnet 103, N-poles and S-poles are alternately provided on the inner peripheral surface of the rotor 102 at an interval of 90 degrees. Further, in case where there are 6 slot-stators 105, teeth 107 around which winding wires (not shown) are wound are formed at an interval of 60 degrees.
In the outer rotor type motor, the rotor 102 is arranged on the outer side of the stator 105, and accordingly, the width in the circumferential direction of each magnetic pole of the permanent magnet 103 (length of arc) is relatively large, as compared with an inner rotor type motor (a motor in which the rotor is arranged on an inner side of the stator). As a result, in the outer rotor type motor, the amount of magnetic flux passing a magnetic pole of the permanent magnet 103 is larger than that in the inner rotor type motor. Thus, in the outer rotor type motor, the amount of magnetic flux passing through the teeth 107 of the stator 104 becomes large in the magnetic circuit including the rotor 102 and the stator 105. Further, a high power magnet of large magnetic flux density has been used in recent years in order to improve a motor performance.
Thus, during the rotation of the rotor 102, when a magnetic flux from the magnetic poles of the permanent magnet 103 (refer to the narrow lines in FIG. 7) passes through a tooth 107 of the stator (refer to A and B in FIG. 7), magnetic saturation is occurred, and in some cases, further passage of the magnetic flux becomes impossible.
Accordingly, even if a high performance magnet is used, its performance cannot be fully achieved. Magnetic saturation can be suppressed by enlarging the size of the teeth 107 of the stator 105, but this approach is not desirable since it causes the size of the motor to increase.